In many machines, seals are provided between rotating and stationary components. For example, in steam turbines, it is customary to employ a plurality of arcuate seal ring segments to form a labyrinth seal about and between the stationary and rotating components. Typically, the arcuate seal ring segments are disposed in an annular groove in the stationary component designed to be concentric about the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. In labyrinth-type seals, the seal faces carry a radially directed array of axially spaced teeth and which teeth are radially spaced from an array of axially spaced annular grooves forming the sealing surface of the rotating component. Alternatively, the rotating component may have a smooth surface in radial opposition to the array of teeth on the seal faces. In any event, the sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface of the rotating component. As a further alternative, a combined labyrinth/brush seal configuration may be used as part of the sealing faces of the segments. The primary seal is therefore accomplished by the brush seal, with the labyrinth teeth serving as back-up seals in the event of a failure of the brush seal.
In a typical installation, the annular groove of the stationary component is dovetail-shaped, having locating flanges directed axially toward one another and defining a slot therebetween. The stationary component is split lengthwise such that the semi-annular dovetail grooves may receive generally correspondingly shaped arcuate seal ring segments. More particularly, the arcuate segments are similarly dovetail-shaped, having a pair of flanges directed axially away from one another for disposition within the dovetail groove and a neck joining the seal face and the flanges of the segment and passing through the slot defined by the locating flanges of the groove. The neck carries the arcuate seal face radially inwardly of the groove when installed.
In this type of seal, the ability to maintain a tight uniform clearance without physical contact between the rotating and stationary components is critical to the formation of an effective seal. If this radial clearance between the seal faces of the segments and the opposing seal surface of the rotating component becomes too large, less turbulence is produced and the sealing action is compromised. Conversely, if the clearance is too tight, the sealing teeth may contact the rotating element, with the result that the teeth lose their profile and tight clearance and thereafter create less turbulence, likewise compromising the sealing action.
Current variable clearance packing rings often use high pressure steam to overcome a radially outwardly directed spring force and move the packing ring segments radially inwardly toward the rotor after start-up. That is, at start-up, the springs locate the packing ring segments in a radially outermost position, affording a large clearance between the sealing faces of the segments and the rotor to accommodate relative movement of the rotor and stationary component, particularly radial excursions of the rotor during start-up. At steady-state, the steam pressure overcomes the spring force and displaces the segments radially inwardly to a small clearance position relative to the rotor.
In an effort to avoid possible hang-up or binding of the seal segments due to uneven friction forces and potentially open up large leakage gaps in comparison with the more conventional fixed or spring backed positive pressure packing ring segments, another form of a variable clearance packing ring seal between stationary and rotating components has been proposed which uses thermal expansion characteristics of the various elements to enable a large seal clearance between the packing ring segment sealing face and the rotating component at start-up, while ensuring a small clearance and minimal inter-segment leakage therebetween at steady-state operation. (See U.S. patent application Ser. No. 09/060,902, filed Apr. 15, 1998, of common assignee herewith). In that application, there is provided a packing ring segment having a lower coefficient of thermal expansion than a seal holder, i.e., the stationary component, typically a turbine diaphragm or packing casing. Also provided is a centering ring having a greater coefficient of thermal expansion than the seal holder. A pair of centering rings are provided on axially opposite sides of the neck of the sealing segments and are supported by the stationary component, for example, by reroundable dowels affixed to the seal holder and projecting in an axial direction into the dovetail cavity. The packing ring segment is supported by the centering rings and each segment is biased radially inwardly by springs acting between the segments and the stationary component. The packing ring segments engage contact surfaces on the opposite ends of the centering rings to maintain a large clearance position at start-up between the seal faces of the segments and the rotary component. In that position, the segment ends are closed.
The centering rings have a greater coefficient of thermal expansion than the seal holder, rotor and packing ring segments. After start-up and as the temperature rises, the centering rings thermally expand in a circumferential direction to a greater extent than the packing ring segment. The contact surfaces, forming the support points for the packing ring segment on the centering rings, are therefore shifted circumferentially to enable the packing ring segment to be displaced radially inwardly. As the machine reaches steady-state operation, the packing ring segments engage the outer surface of uniform diameter of the centering rings, thus assuring concentricity of the packing ring segment sealing surfaces relative to the rotary component, enabling a small uniform clearance between the sealing faces of the segments and the rotary component and opening end gaps between the seal segments. The result is a uniform clearance seal that retains its labyrinth tooth geometry during start-up and a steady-state operation and has minimal bias leakage.